As described in patent application Ser. No. 13/473,132 filed May 16, 2012 incorporated herein by reference, it is desirable to provide a thin structure for an antenna embedded in an armor panel and more particularly to provide a parasitic bowtie dipole on top of the armor layer so that when driving the antenna there are no apertures in the armor which would degrade performance. In one embodiment, the aperture-less embedded antenna system includes a direct fed dipole on the underneath side of the armor layer such that the armor layer is not pierced. There is an identical dipole on the top of the armor layer that is parasitically fed by the driven dipole. In one embodiment the dipoles are in the form of bowties.
As described in the above identified patent application, it is desirable to replace antennas such as whip antennas, conventionally attached extending from tanks, armored vehicles and the like, with broadband antennas that are conformal to an outer surface of the vehicle itself.
For example, having a forest of antennas that extend from the armored vehicle is undesirable because they are susceptible to damage and attack. It is therefore desirable to be able to provide an antenna system which is embedded within the armor so that the armor protects the embedded antenna both against explosive attacks and ballistic penetration. It is also desirable to eliminate the need for antenna whips, or similar configurations, which are easily damaged by explosive charges, thereby precluding communication with the vehicle.
It is noted that the thin structure of the prior art armor panels presents the greatest challenge to similar antenna design. Whether the panel is metal backed or is mounted on a metal vehicle, the close proximity of a conductive surface to a radiating bowtie dipole creates a ground plane that is too close to the bowtie dipole. As will be appreciated for traditional antenna design, the ground plane is spaced at least a quarter wavelength away from any driven bowtie dipole. However, when dealing with armor for vehicles, such as tanks, the spacing between the ground plane and the driven bowtie dipole of the antenna is on the order of hundredths of a wavelength.
While initially thought that this limitation would be a disqualifying factor in similar antenna designs, it has been shown that a thin antenna structure can be created which does not rely on deep cavities behind the bowtie dipoles. However, as described in the above patent applications, it has also been found that the close spacing, as well as other factors, disadvantageously limit bandwidth and gain. Indeed, this close spacing has also been found to result in non-optimal voltage standing wave ratios (herein after referred to as VSWR) across desired bandwidths, for instance between 225 MHZ and 450 MHZ.
Examples of these deep cavity structures are described in U.S. Pat. No. 6,833,815 which relates to Cavity Embedded Meanderline Loaded Antennas. In this patent, the antenna is described as a conformal antenna which is cavity-backed. According to one embodiment of this disclosure, a bowtie dipole is utilized, with the distal ends of the dipole being coupled to surrounding metal utilizing a meanderline structure.
The question becomes how one can better configure such dipole antenna into a thin structure for use with armor plates without disadvantageously limiting bandwidth and gain.